As is known, the internal-combustion engine of a motor vehicle is connected via a belt drive to one or more auxiliary devices, such as, for example, an electric machine and a compressor of an air-conditioning system.
Known to the art are so-called “start-stop” systems, in which the electric machine is reversible, of the motor-alternator type, and is able to function both as an electric generator driven by the internal-combustion engine and as starting motor, which drives the internal-combustion engine. During operation of the drive according to these two modalities, the torque that is impressed on the drive changes sign during a transient in which the tensioned belt run and the slack belt run of the belt are reversed. In the drives of the type described above, the tensioning of the belt is entrusted to a two-armed tensioner designed to co-operate simultaneously with two distinct belt runs of the belt.
Two-armed tensioners are known which comprise a pair of substantially rectilinear arms, each having a first end pivoted on a common axis set within or without the closed path of the belt, and a second end bearing an idle pulley that co-operates with the back surface of the belt itself. Known two-armed tensioners further comprise a spring, which acts by bringing the arms closer to one another and exerting a tensioning force on the belt.
Known two-armed tensioners present some drawbacks, due to the criticality of the sizing of the spring, which is subjected to conditions of operation that vary within a very wide range of values and to the relative slowness of the response to the transient of reversal of the torque.
In particular, in the two modes of operation described, the pulleys are subject to translations, due to the reversal of the tensioned belt runs, which can reach high values especially when the belt has increased its own total length on account of the slackening that occurs typically in a belt towards the final stage of its service life.
Furthermore, the transient of reversal of the torque has a very short duration, and conventional two-armed tensioners tend to react to the swapping-round between the slack belt run and the tensioned belt run with a delay that causes temporary slipping and consequent damage to the belt.